1. Field of the Invention
The present invention relates to a thermosetting copolymer, a silicon carbide-based fiber and processes for producing the same. This thermosetting copolymer is a copolymer of a thermoplastic silicon-containing polymer and perhydropolysilazane or polyborosilazane, and since it exhibits thermosetting property, is able to be converted directly to ceramics by pyrolysis and is useful as precursor polymer of ceramic fibers, ceramic coatings, ceramic binders, etc. This silicon carbide-based fiber is a novel inorganic fiber comprising Si, N and C as the major components and silicon carbide as the main phase, and since they have a high strength, modulus and thermal resistance, is useful as a reinforcement of various composite materials.
2. Description of the Related Art
Silicon carbide is excellent as a structural material and polycarbosilanes, polysilastyrenes, polysilanes, etc., are known as precursor polymers of the silicon carbide (Ceram. Eng. Sci. Proc., 97-8, 1988, p 931-942; and Japanese Unexamined Patent Publication (Kokai) Nos. 51-126300, 52-74000 and 52-112700). Silicon carbide fibers formed by spinning a polycarbosilane followed by curing and firing are known (J. Am. Ceram. Soc., 59, 1976, p 324-327; and Chem. Lett. No. 9, 1975, p 931-934).
Also, polytitanocarbosilane, polyzirconocarbosilane, polydisilylazane, etc., are known as precursors of silicon carbide-based ceramics such as SiC--TiC (U.S. Pat. Nos. 4,340,619, 4,321,970 and 4,482,689 and Japanese Examined Patent Publication (Kokoku) Nos. 2-33734, 2-33733, 61-58086 and 62-61220).
Silicon carbide-based fibers obtained from polytitanocarbosilane and polyzirconocarbosilane are also known (Japanese Unexamined Patent Publication (Kokai) Nos. 56-9209, 57-106718, 57-106719 and 60-99004 or U.S. Pat. Nos. 5,085,819; 4,515,742; 4,063,229; 4,342,715 and 4,399,232).
Polysilazanes and polyborosilazanes, however, are known as precursors of silicon nitride and silicon nitride-based ceramics, and the inventors have disclosed such polymers (Japanese Examined Patent Publication (Kokoku) No. 63-16325). Si--N--C--O--based ceramic fibers obtained by spinning an organic silazane followed by curing and firing are known (Japanese Unexamined Patent Publication (Kokai) Nos. 49-69717, 49-20206 and 57-200210; U.S. Pat. No. 4,482,669; and J. of Appln. Poly Sci., Vol. 27, 3751-61, (1982). Also, we disclosed silicon nitride fibers produced from perhydropolysilazane and containing a lesser amount of carbon (Japanese Unexamined Patent Publication (Kokai) No. 62-125015 or U.S. Pat. No. 4,818,611).
Since the above precursors for silicon carbide or silicon carbide-based ceramics are thermoplastic, a curing step is essential for producing a ceramic product from these precursor polymers. The curing process may be any of known such as thermal oxidation, steam treatment, .gamma.-ray irradiation, electron beam irradiation halogen treatment, ozone treatment, etc. But use of radiation is complicated and dangerous, and further, is inefficient and also causes contamination with impurities. Particularly, oxygen introduced by the thermal oxidation damages the high temperature properties of the obtained ceramics. Further, the ceramics produced from these percursor polymers contain much free carbon and the perhydropolysilazane does not provide silicon carbide ceramics.
Silicon nitride and silicon nitride-based ceramics are candidates for high temperature structural material having an excellent strength and toughness, etc. Since silicon carbide provides a structural material having a higher strength, composites of the silicon nitride or silicon nitride-based ceramics with silicon carbide are effected to provide ceramics having more excellent properties. Further, the micro structure of the derived ceramics can be controlled by doping adequate amount of boron.
Therefore, the object of the present invention is to convert a thermoplastic polymer to a thermosetting polymer, to thereby provide a polymer which can be converted into ceramics without a curing process, more particularly, to provide a polymer useful as a precursor of composite ceramics of silicon carbide and silicon nitride. Another object of the present invention is to provide silicon carbide-based ceramic fibers having a lesser amount of oxygen and free carbon.